Using the `nix-env` command is far more robust. It also has the
benefit that if the per-user `profiles` and `gcroots` directories do
not exist then they will be created with the correct permissions.
Because of the second point this commit also removes the `mkdir` step
of the installation instructions.
PR #1239Closes#474, #948, #1091
The format script can be used to automatically format the Nix source
files and also verify that the files are formatted using the `-c`
command argument.
At the moment some files are exempt from the formatting to avoid
causing merge conflicts in active pull requests.
Finally, update the contribution guidelines to note that `nixfmt`
should be used.
This avoids the uncontrollable nature of fetching the tarball as part
of the evaluation. Instead the user can decide when to update and also
perform rollbacks, if necessary.
